Generally, in view of some reasons (e.g. safety), an additional airflow passage structure is extended from a small-size fan. FIG. 1 is a schematic cross-sectional view illustrating a fan assembly according to the prior art. As shown in FIG. 1, the fan assembly 1 comprises an airflow passage structure 10 and a fan 11. The airflow passage structure 10 is substantially a straight tube-shaped structure with a uniform internal diameter D. An airflow channel is defined by the inner surface 101 of the airflow passage structure 10. The airflow passage structure 10 is installed at an airflow inlet 110 of a fan 11. The use of the airflow passage structure 10 may change the characteristics of the fan, comply with the layout size of the fan or increase the safety of the fan.
Since the direction of the airflow entering the airflow channel of the airflow passage structure 10 is not completely parallel with the inner surface 101 of the airflow passage structure 10, a portion of the airflow possibly stagnates within the airflow channel. That is, since some stagnation zones are possibly formed in the vicinity of the inner surface 101 of the airflow passage structure 10, a portion of the airflow whirls within the airflow channel. Under this circumstance, the performance of the fan is deteriorated, and the noise resulted from the fan is increased.